The present invention relates to a device for improved artificial sensibility.
The human hand is a grip organ that is often characterized by good grip power as well as adequate capacity for fine motor precision movements. The hand is also a sense organ that has an extremely well developed sensory function. The sensibility of the hand includes not only the ability to feel, for example, pressure, touch, vibration, temperature changes but also the ability to perceive the form, surface and texture of objects that are touched without requiring the person to see the objects, so called stereognosis. The function of the hand is to a large extent dependent on the sensory feedback system that is accomplished by sensibility. A hand without sensibility is a hand that does not function well.
There are many situations where lack of sensibility constitutes a major hindrance for the hand function and where some kind of artificial sensibility would be of great importance. A number of such situations are reviewed below.
Following amputation injuries of a forearm or an upper arm level, the patient can have good use of an artificial hand, i.e., a hand prosthesis that can replace the amputated hand. Such a hand prosthesis can be fixed to the amputated stump by the use of a sleeve enclosing the remaining part of the forearm or by titanium fixtures, such as according to the Br{dot over (a)}nemark model, which are implanted into the remaining parts of the skeleton. The prosthesis can be of merely a cosmetic nature without any possibility of movement or of a functional type with the possibility of active voluntary motion. The latter case refers to muscle-controlled myoelectric prostheses. These prostheses are controlled by electric activity, released by matter of will, in the muscles that remain in the amputation stump. By using the surface electrodes in the sleeve enclosing the amputation stump, such impulses in the extensor or flexor muscles are recorded and transformed for control of a motor in the prosthesis which can open or close the prosthetic hand.
Myoelectric hand prostheses can be very useful but many patients use their prostheses only to a limited extent or not at all. The direct reason is often that the prostheses lack sensibility and the patients thus lack the feedback function in the system. Many patients are dependent on visual observation of every movement in the prosthesis in order for the vision sense to, to some extent, compensate for the lack of sensibility. A system which enables real sensibility in such hand prosthesis, would dramatically improve its function.
Severe transection or crunch injuries in the arm or hand include lesion of one or several nerve trunks. For instance, transection of one of the major nerve trunks on the volar part of the wrist, such as the median nerve, results in total sensory lose within the major part of the hand. Such an injury results in major disability since the hand functions are very much impaired due to the sensory lose. Following surgical repair of the nerve, new nerve fibers grow in the hand but because of disorientation of nerve fibers and incomplete innervation, the sensibility often remains impaired in the hand. Following injury of nerve trunks at the wrist level, it takes about six months before there is any useful sensory recovery in the hand. During these months, when there is a complete sensory loss of the hand, the patient has great difficulties in exercising the hand since there is no sensory feedback. A system for artificial sensibility during this period would to a large extent facilitate the post-operative training and thus lead to a more rapid rehabilitation.
There are a large number of neurological diseases that may result in impaired sensibility or total sensory loss in the hand. In polyneuropathies, there may be such a sensory impairment in many parts of the body including the arms and hands. Such a neuropathy may exist in, for example, diabetes, alcohol abuse or impaired kidney functions. Impaired sensibility, can be present also in multiple sclerosis and stroke impaired patients. In industrial societies, sensory impairment due to long-term use of handheld vibrating tools is a major problem. In a global perspective, leprosy or lepra is the most obvious example of how sensory impairment results not only in impaired hand function but also in injuries and infections which may lead to spontaneous amputations of fingers, toes and other body-parts.
In these situations, restitution of sensibility in the hand and other body-parts could lead to a major advantage in terms of improved function and could also help to prevent injuries to the body.
The description, given above, has primarily addressed problems related to the hand and the upper extremity. Also, lower extremity sensory loss may lead to major problems. In diabetes, sensory loss in the foot may lead to wounds, infection, necrosis and amputation. In amputation injuries, prostheses may be very useful but because of lack of sensibility in the prostheses their use may be limited.
Artificial sensibility enables lost or impaired sensibility to be replaced by an alternative system for sensory feedback. To achieve such sensibility, the following is required: 1) Detection of a sensory stimuli, e.g., pressure or vibration, 2) Processing and transmission of the signal resulting from such a stimuli; and 3) Sensory perception based on the transmitted signal.
Registration of a stimulus requires some type of a sensor. For example, pressure piezo-resistive membranes have been used in, for example, the grip members of industrial robots. Also, in experiments on humans, piezo-resistive membranes applied to the fingertips of hand prostheses or denervated hands have been used. A pressure, applied to such a membrane, initiates an electric signal which is proportional to the applied pressure.
The electric signal can be processed and used for application of electric or vibro-tactile proportional stimuli to intact sensible skin on a suitable area of the same extremity or at some other part of the body. This principle, however, has major drawbacks. For example, due to rapid adaptation of the skin to such a stimulus, the skin becomes less sensitive. In addition, such a principle gives an unnatural alternative stimulus which can not be used for identification of the surface or structure that is touched.
One object of the present invention is to create possibilities of feeling with the help of the hearing sense to replace vibrotactile sensory stimuli with acoustic stimuli. The invention creates possibilities to, by using the hearing sense, identify the character of the surface and texture which is touched and to thereby achieve a descriptive functional sensory perception. The invention is based on a new principle for artificial sensibility based on a sense substitution, i.e., the use of an alternative sense to replace a lost sense.
Various types of sense substitution exist in daily life. For example, a blind person can read in Braille with the fingertips so that the vision is replaced by sensibility. A deaf person can read the movements of the lips of the contrahent, so that the hearing sense is replaced by vision, and can also feel and utilize the vibrations in the larynx of the contrahent so that the hearing sense is replaced by the sensibility.